Purpose: To illustrate the measurement
of the mass of a planet using Kepler's third law.

The software provides a view of Jupiter at four magnifications (actual
Voyager images are used), along with a highly accurate ephemeris program
that draws the four Galilean satellites in their proper positions relative
to the planet at any time. Students make observations of Jupiter and its
satellites at regular intervals over a period of several weeks and, by
graphing the separation of each moon from Jupiter versus time, they measure
the period and radius of each satellite's orbit. This is sufficient information
to derive the mass of Jupiter. Students use the mouse cursor to identify
the moons and to measure distances. A cloudy night feature provides some
incompleteness to the data. Instructor-settable options set the percentage
of "cloudy" days, suppress the automatic moon identification, and suppress
the automatic calculation of distance from the planet.